For Multiple-Input-Multiple-Output (MIMO) wireless communication, multiple antennas are used at both the transmitter and the receiver. Because MIMO increases data throughput without additional bandwidth or transmit power, MIMO is becoming increasingly popular in wireless communications standards. One issue with MIMO is that transmission quality degrades in Line-of-Sight (LOS) and poor-scattering environments. More specifically, MIMO relies on an uncorrelated transmit channel resulting from a multi-path environment. However, in LOS and poor-scattering environments, transmit channel correlation increases. As a result, transmission quality decreases.
In order to address this issue, current MIMO transmitters typically operate in a closed-loop configuration. In the closed loop configuration, the MIMO transmitter performs transmit channel based precoding prior to transmission to a MIMO receiver. One such example is precoding for MIMO as defined by the Long Term Evolution (LTE) standard. In LTE, the MIMO receiver estimates the transmit channel and selects one of a number of predefined precoding matrices based on the estimate of the transmit channel. The precoding matrices are unitary, and the selected precoding matrix is one which will maximize capacity based on the estimate of the transmit channel. The MIMO receiver then provides feedback to the MIMO transmitter, where the feedback includes the selected precoding matrix or a reference to the selected precoding matrix in a codebook stored by the MIMO transmitter. The MIMO transmitter then applies the selected precoding matrix to a signal prior to transmission. However, since the transmit channel changes frequently and the precoding matrix is selected based on the transmit channel, the transmit precoding matrix must also be updated frequently. More specifically, the MIMO receiver must estimate the transmit channel, select a precoding matrix, and feed back the selected precoding matrix or a reference to the selected precoding matrix to the MIMO transmitter frequently in order to reflect changes in the transmit channel. Using MIMO as defined by the LTE standard as an example, a new precoding matrix is preferably selected and fed back on the order of every 4-5 sub-frames.
However, one issue with the transmit channel based precoding scheme of current closed loop MIMO transmitters is that the transmit channel based precoding scheme is not suitable for high-order MIMO. More specifically, as the order (i.e., number of antennas) increases, the complexity of the transmit channel precoding matrix and therefore the overhead required for feedback of the transmit channel based precoding matrix also increases. As such, there is a need for a precoding scheme for high-order MIMO that improves transmission quality and minimizes or substantially reduces the overhead required for feedback.